Photopolymerizable compositions and photopolymerizable inkjet inks using (meth)acrylic acid esters have been widely known (see PTL 1).
However, many of monomers used in conventional photopolymerizable inkjet inks are toxic. Especially, most of (meth)acrylic acid esters, which are readily available with low cost, have high toxicity in terms of skin sensitivity, which causes allergy reactions with skin upon contact with them. Conventional art has not yet provided any solution to this problem.
Based on the studies conducted so far, the present inventors have found a few (meth)acrylic acid ester and (meth)acryl amide, which have no problem in skin sensitization. As for one of examples thereof, proposed is an inkjet ink containing methacrylate as a main component, as methacrylate is lower in toxicity in terms of skin sensitization than acrylate, as disclosed in Japanese Patent Application No. 2010-278177, and PTL 2. In order to further improve the performances, it was been desired to improve a strength of a cured coating film to enhance durability thereof against abrasions or the like, and to reduce a viscosity so that it can be ejected by inkjet without any problem even when an ink increases its viscosity as a result of formulated various materials, such as a pigment, and additives, therein. To reduce viscosity of an ink, water can be blended therein, and a photopolymerizable aqueous inkjet ink, to which water is blended, has been known. As described later, however, such ink is not penetrated and dried on a non-absorbent base. Use of such ink requires a step for evaporating water in order to increase speed or attain efficiency of a process. Moreover, a heat source is desired to be provided. Therefore, it is not preferable in view of energy saving. In the case where the photopolymerizable composition is used as a coating material used for brush coating, an organic solvent can be used therein. However, the organic solvent is evaporated and released in the atmosphere. In view of a possible influence to the environment, use of the organic solvent is preferably as little as possible. In the case where the photopolymerizable composition is used as an inkjet ink, if an organic solvent blended therein is highly volatile, the ink in a nozzle, through which the ink is jetted, increases its viscosity due to vaporization of the organic solvent, which may cause a problem in jetting. Therefore, such ink is not preferable.
In view of these points mentioned above, the present inventors have invented technologies disclosed in Japanese Patent Application Nos. 2012-46301 and 2012-113970. Considering an object for improving a coating film strength under the situation that polymerization reactivity is more or less sacrificed as materials are formulated in view of reduction in skin sensitization, it is very important to further improve scratch resistance of a surface of a coating film using a method other than acceleration of a polymerization reaction for the purpose of protecting surfaces of housing or molded articles using the photopolymerizable composition, which is an important task together with a task for attaining excellent appearance, such as high glossiness, achieved by forming uniform a surface of a coating film without any irregularities.
Moreover, if a solid coating film is produced using the photopolymerizable composition, not by printing a certain image pattern, not only an inkjet system, but also a method, such as spray coating, and brush coating, can be employed. It is however difficult to provide a coating film, which has no problem of skin sensitization, and has high strength and excellent appearance, regardless of a coating system.